1. Field of the Invention
The present invention relates to gas generation retarded aluminum powders for oil field cements, and more particularly, to methods of forming and using essentially dry retarded aluminum powder capable of being dry blended with cement.
2. Description of the Prior Art
In the completion of an oil, gas or water well, the annular space between casing disposed in the wellbore and the surrounding earth formations is cemented whereby the casing is bonded to the formations and the annulus is sealed to prevent formation fluid flow therethrough. Most often, a cement slurry is pumped down the inside of the casing and back up through the annular space outside the casing to the desired location therein. Once placed, the cement slurry is allowed to set into a hard mass.
In some wells, one or more of the zones adjacent the cemented annulus contain connate gas under substantial pressure which can enter the annular space after the cement slurry is placed therein, resulting in gas leakage. The gas can leak through the cemented annular space to the surface or to other subterranean zones as the cement slurry sets thereby forming a permanent flow channel or a highly permeable cement after final set. Such gas leakage is detrimental and often dangerous, and usually requires expensive remedial cementing operations to correct.
In order to prevent gas leakage problems, oil field cements which include gas generating agents have heretofore been developed and used. For example, U.S. Pat. No. 4,304,298 issued on Dec. 8, 1981 discloses a cement slurry containing a stabilized, dispersed gas therein whereby the slurry is compressible. The presence of the gas in the slurry and the compressibility thereof prevents gas under pressure from passing into and around the cement prior to the time the cement has set or gelled sufficiently to prevent upward gas percolation. The entrained gas, by virtue of its compressibility, reduces the magnitude of the pressure drop decrease resulting from slurry volume reduction. The preferred method of forming gas in the cement slurry is to include a metal, preferably aluminum powder, in the slurry which reacts with the aqueous alkaline solution therein to yield hydrogen gas in the slurry.
As indicated in the above-mentioned patent, it is often desirable to retard the reaction rate of aluminum powder mixed with oil field cements so that the generation of hydrogen gas therein is delayed. When unretarded aluminum powder is mixed with a cement slurry, the reaction of the aluminum with the aqueous solution in the slurry to produce hydrogen gas occurs rapidly, and in many instances, the hydrogen gas can be released too quickly. For example, if the hydrogen gas is released in the cement slurry prior to placing it in the annulus, a large part of the effectiveness of the slurry in preventing gas leakage can be lost. In addition, hydrogen gas is extremely explosive and its generation at inappropriate times, such as during surface mixing operations, can be dangerous.
Another problem which can occur in the use of aluminum powder for gas generation in cement slurries is the reaction rate retardation brought about by oxidization. The retarding effect of aluminum powder surface oxidization caused by exposure of the powder to the air can be so strong that no reaction takes place or the reaction that does take place is delayed too long and is not effective for controlling gas leakage.
In order to overcome these problems, a variety of coatings and chemical retarders have heretofore been developed and used in association with aluminum powder in oil field cements. For example, U.S. Pat. No. 4,367,093 issued Jan. 4, 1983, discloses a cement composition and method of cementing whereby the rate of release of hydrogen gas formed from aluminum powder is controlled by including a chemical inhibitor in the cement. While this and other similar methods of controlling or inhibiting the reaction of aluminum powder in cement have been used successfully, they have been expensive or require that the aluminum powder be premixed with one or more inhibitor chemicals in a carrier liquid followed by combining the liquid mixture with a cement slurry.
By the present invention, methods of forming and using an essentially dry hydrogen gas generation retarded aluminum powder which is protected from oxidization and which can be dry blended directly with cement are provided.